You're underestimating how huge the surface area of the sphere these satellites are orbiting, and how little material they have. Even if you somehow blow them all up, they won't shield you from ICBMs.

And if the debris field somehow was dense enough to provide such a shield it would block out enough sunlight to cause the death of most life on the planet I suspect.

On the positive side, a solution for global warming.

If you blow up all starlink satelites, the resulting debris will start taking out other satelites.

If you blow up all the Starlink satellites, the little pieces start reentering the atmosphere most preferentially in very short order.

Kessler syndrome is more complicated than just "satellite debris bad". At the altitudes the Starlink satellites are at, it is a near non-concern. There is a causal link there... we can afford to put up lots of little satellites at that altitude for that exact reason, and that's exactly why they are where they are. That it's also less energetically expensive to launch is just a bonus.

It seems that there's a threshold that would have to be hit to trigger that process. Do we have any idea whether starlink satellites, whether intact or as debris, would be anywhere close enough to that threshold to trigger the runaway process?

It could be true that the phenomenon exists conceptually, but that were nowhere near remotely close to triggering it, and that people are vastly underestimating the size of space.

… … … you mean it will cause Kessler Syndrome?

ICBMs don't stay in space for long enough to be impacted.